1. Field of the Invention
The present invention relates to electrically controlled hydraulic systems for operating machinery, and in particular to determining in which one of a plurality of hydraulic fluid metering modes the system should operate at any given time.
2. Description of the Related Art
A wide variety of machines have members which are moved by a hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine user. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of electrohydraulic valves, such as those driven by solenoids. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates sophisticated computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator and fluid flow back from the actuator to a reservoir is governed by an assembly of proportional solenoid operated spool valves. To control a cylinder-piston type hydraulic actuator for example, four solenoid valves are connected in the legs of a Wheatstone bridge with the supply line from the pump and return line to the reservoir coupled to two opposite bridge corners and two cylinder chambers connected to the other two corners, as described in U.S. Pat. No. 6,880,332. By selectively operating different pairs of the valves, fluid is conveyed to and drained from the cylinder chambers to extend and retract the piston rod. The amount that each valve opens is directly related to the magnitude of electric current applied to the solenoid coil, thereby enabling proportional control of the hydraulic fluid flow.
When an operator desires to move a member on the machine a joystick is operated to produce an electrical signal indicative of the direction and desired rate at which the corresponding hydraulic actuator is to move. The faster the actuator is desired to move the farther the joystick is moved from its neutral position. A control circuit receives a joystick signal and responds by producing a signal to open the pair of valves associated with the direction of the desired motion.
The aforementioned U.S. patent describes a velocity based hydraulic control system having a plurality of different metering modes which are selected to drive the actuator in the intended direction. The metering modes utilize fluid from different sources in the system and consume various amounts of power to operate the pump. Therefore, some metering modes are more energy efficient than others. However, a particular metering mode may only be available under certain operating conditions, such as requiring specific pressure relationships among sections of the hydraulic system.
The fundamental metering modes in which fluid from the pump supply line is supplied to one of the cylinder chambers and drained to the reservoir return line from the other chamber are referred to as “standard metering modes”, specifically a standard extension metering mode or a standard retraction metering mode. A hydraulic system also can employ regeneration metering modes in which fluid draining from one cylinder chamber is fed back through the valve assembly to supply the other cylinder chamber. In a regeneration metering mode, the fluid can flow between the chambers through either the corner of the valve bridge connected to the supply line, called “high side regeneration”, or through the valve bridge corner coupled to the reservoir return line in “low side regeneration”. In cross function regeneration metering modes, fluid exiting under pressure from one hydraulic actuator is routed, either through the supply line or the return line, to power another hydraulic actuator. The regeneration metering modes employ fluid being exhausted from a hydraulic actuator in place of fluid from the pump thereby saving energy than otherwise is required to drive the pump.
An electronic controller for the hydraulic system monitored the operating conditions that were used to determine the metering mode and automatically selected the most efficient mode that was functionally available. When the operating conditions changed so that it was advantageous to use another metering mode than that which was currently active, the system switched directly to the more efficient metering mode. This worked effectively in many situations, such as when a sharp load change occurred, for example upon the bucket of an excavator hitting the ground. However, abrupt metering mode transitions did not work well in other situations, such as when the excavator bucket was elevated in the air or when a telehandler boom was extending. In these latter situations, the abrupt metering mode transition often produced a jerk in the machine motion, which upset the machine operator who erroneously believed that the machine was malfunctioning. The prior solution involved restricting the occurrence of metering mode transitions to only when a sharp load changes took place. However, this dramatically limited the efficiency derived from having multiple metering modes.